CALGEM: Taking the Measure of California's Greenhouse Gas Emissions

To play its part in reducing the consequences of global warming, the state of California has embarked on a trailblazing effort to reduce its emissions of greenhouse gases (GHGs). Assembly Bill 32, recently passed by the California legislature and signed into law by Governor Arnold Schwarzenegger, requires the state to substantially reduce GHG emissions by the year 2020—and, by the summer of 2008, to develop a plan for accomplishing this.

Transportation and power plants are the principal sources of greenhouse gas emissions in California, but there are many other natural and artificial sources as well.

The bill requires California to reduce its carbon emissions to 1990 levels by 2020, a reduction of 14 percent from 2004 levels. By the Governor's executive order, carbon emissions must be reduced to 80 percent below 1990 levels by 2050. Given that California has the fifth largest economy in the world—with total greenhouse gas emissions estimated at around 500 million metric tons in 2004, the second largest of any state—both these targets require substantial reductions. Because California pioneered many of the current technologies for energy efficiency and renewable energy, it is reasonable to expect that the state can meet this challenge.

To prove that greenhouse gas reductions are actually taking place, it will be necessary to monitor emissions regionally. Scientists at the Environmental Energy Technologies Division of Lawrence Berkeley National Laboratory (Berkeley) recognized this need several years ago and began studying the problem. They are now taking the first steps toward creating a monitoring network with a pilot project called CALGEM: the California Greenhouse Gas Emissions Project.

Measuring net exchange of greenhouse gases

In 2003, a group led by Marc Fischer in EETD began an exploratory project with the California Energy Commission to develop a method of quantifying GHG emissions on regional scales. Fischer, William Riley of the Earth Sciences Division, and Shaheen Tonse of EETD designed a statewide network of atmospheric measurements to monitor carbon dioxide (CO2) emissions, described in a report titled "Development of an Implementation Plan for Carbon Monitoring in California."

Using data from existing sources, plus computer models for atmospheric gas transport, Fischer's group predicted variations in CO2 concentrations across California. Atmospheric CO2 is affected by both anthropogenic (human-caused) emissions, mostly the combustion of fossil fuels, and by the net ecosystem carbon exchange.

Computer-simulated maps developed at Berkeley Lab for the California Energy Commission show predicted CO2 concentrations in the surface layer of atmosphere from net ecosystem carbon exchange and fossil fuel combustion. During the summer, CO2 is actively removed from the atmosphere in some locations by photosynthesis, and released in others through respiration, as soil microbes decompose organic matter. During the winter ecosystem processes show different patterns, reflecting a combination of cooler temperatures, moister conditions, and the action of different plant groups. Fossil fuel CO2 emissions, concentrated in urban areas across the state, are more constant across the seasons. (Courtesy landandseaimages.com)

In an actively growing ecosystem during the day, more CO2 is removed from the atmosphere by photosynthesis than emitted back to the atmosphere by plant and soil respiration. At night, however, CO2 is emitted back to the atmosphere by respiration. The lower atmosphere responds to additions and removals of CO2 on daily to seasonal time scales, acting like a large mixing tank with a varying volume.

The team predicted variations of midday CO2 from ecosystem exchange and fossil fuel emissions in the lower 100 meters (about 300 feet) of the atmosphere for two seasons. They showed not only that ecosystem and fossil fuel signals are large enough to measure, but also that these signals are sufficiently different in spatial and temporal variation that, by measuring CO2 at many sites, human contributions could be identified. With the exploratory study as proof of concept, the Berkeley Lab group launched CALGEM.

"We will design an atmospheric measurement network to quantify greenhouse emissions, and estimate whether the atmospheric measurements are likely to provide sufficient accuracy and precision to tell whether GHG control strategies are working or not," says Fischer.

Greenhouse gases are many — and many are more potent than carbon dioxide

Carbon dioxide is not the only greenhouse gas. Many others, including methane (CH4), nitrogen oxides, and halocarbons, also contribute to global climate warming. These non-CO2 greenhouse gases are estimated at 15 percent of the total emissions from the state of California.

Methane, CH4, is the largest of the non-CO2 GHGs. CH4 is mostly emitted by microbial processing of feed in livestock, and as a byproduct of the breakdown of organic waste materials in landfills. Natural gas is predominantly CH4; its delivery and use in industrial processes and transportation emit lesser amounts of methane, as does periodical flooding in agriculture and decomposition of organic material in natural wetlands. Methane's concentration in the atmosphere is now three times higher than it was in the pre-industrial era.

"The problem is that the emissions of CH4 and other non-CO2 GHG gases are poorly quantified," says Fischer. "Uncertainties range from 25 percent to a full order of magnitude." The emissions of these gases are not as large that of CO2, but taken together they have an effect on climate change that is comparable to total CO2 emissions. Accurately estimating their emissions over time is therefore crucial to the effort to measure the effectiveness of GHG reduction programs.

A network of measurements

Fischer, Riley, and Pieter Tans, of the National Oceanic and Atmospheric Administration's (NOAA's) Environmental Science Research Laboratory, will design a network of instrumentation to measure non-CO2 GHGs in California. Atmospheric measuring techniques will be demonstrated at a subset of the sites in the final CALGEM network.

Two tall-tower sites, the Sutro Tower in San Francisco and a broadcast tower in the Central Valley near Sacramento, will be equipped with instruments for the CALGEM project.

Ground-based measurements will include continuous monitoring and flask sampling at tall towers. Dedicated instruments at one site will continuously measure CH4 and CO2, plus carbon monoxide (CO) as a tracer of combustion. Flasks will be collected twice a day, which NOAA will analyze for a host of GHG species. Aircraft and satellite remote sensing will provide estimates of the total amount of GHGs in the atmosphere.

Beyond the GHG measurements, one tower will also take measurements of radon (222Rn) to monitor the rate at which GHGs are diluted in the atmosphere. Radon is a naturally occurring radioactive gas with a half life of 3.8 days. The short half life makes 222Rn an excellent tracer of atmospheric mixing, and a tracer of the origin of atmospheric air masses. For example, air masses coming to California from the ocean have much less radon gas than air masses coming from land. Similarly, air near the land surface has more 222Rn than air in the upper atmosphere.

The 222Rn measurements provide scientists with a tool to quantify how much contact a given air mass has had with the land surface, and therefore the change in GHG concentration expected from a given amount of emission. "Together, these measurements will provide an unparalleled tool for monitoring trends in atmospheric GHG concentrations in California," says Fischer.

Computer modeling to design a better network

Possible locations for the complete regional greenhouse gas measurement network include the San Francisco and Sacramento towers equipped by Berkeley Lab, existing measurement sites operated by other institutions on the North Coast and near San Diego, and several tower locations in the Central Valley and near Los Angeles that could be adapted to provide a complete measurement network in the future. Increased aircraft sampling and satellite remote-sensing measurements will supplement coverage from these ground stations.

To interpret the measurements and to plan an expanded network, the research team is undertaking a parallel effort in data analysis and modeling. Using initial measurements, this part of the work will quantify surface emissions and figure out which expansions to the network would best minimize uncertainty in future estimates.

"As in the initial exploratory project on CO2," says Fischer, "we'll combine current 'bottom-up' models of surface greenhouse-gas emissions with regional-scale models of atmospheric transport, to predict atmospheric GHG concentrations for different sources in California."

The mathematical procedure adds noise to the model predictions to produce "pseudodata" of GHG concentrations, thus introducing uncertainties like those due to instruments or to imperfect knowledge of atmospheric transport of greenhouse gases. The scientists then calculate best estimates of surface emissions using "inverse-model" methods, which match the measured data as closely as possible to the noisy pseudodata.

Finally, the CALGEM team will design a future measurement network that reduces uncertainty in emission estimates, using pseudodata calculations run on many different network designs. The completed project will provide several useful results to the people of California:

"First the measurements will represent the beginning of a long-term record of GHG concentrations, representing California's contribution to global climate change," Fischer says. "Second, the modeling and analysis will provide an initial estimate of the current level of GHG emissions at the regional level, and provide recommendations for a more complete monitoring network."

As California implements reduction programs to achieve the goals of Assembly Bill 32, CALGEM will enable California to estimate how well the programs are working to reduce greenhouse gas emissions.